a) Field of the Invention
This invention relates to a flow control system suitable for use in controlling the rate of a flow through a hydraulic circuit for performing load sensing control, for example, through a hydraulic circuit for driving actuators of a hydraulic work vehicle.
b) Description of the Related Art
A hydraulic control system for an apparatus with plural actuators driven by an oil pressure at the same time, for example, for a work vehicle such as a hydraulic excavator is equipped with a hydraulic pump, plural actuators driven by pressure oil fed from the hydraulic pump, and plural valve devices for controlling the flow rates of pressure oil to be fed from the hydraulic pump to the respective actuators. Known as hydraulic control systems of this type include, for example, load sensing control for controlling the delivery pressure of a hydraulic pump in response to a load pressure. One example of the load sensing control is disclosed in WO 90/00683. According to this conventional technology, there is provided pump control means for controlling the displacement of a hydraulic pump so that the delivery pressure of the hydraulic pump becomes higher by a predetermined value than a maximum load pressure to plural actuators. This pump control means is constructed of flow rate control valves provided with variable restrictors for changing the openings of plural valve devices in accordance with control signals from a control lever device, respectively, and pressure compensation valves (auxiliary valves) arranged on upper sides and in series with the respective variable restrictors to control pressure differences across the variable restrictors, respectively. By controlling the pressure differences across the respective variable restrictors with the corresponding pressure compensation valves, it has made it possible, upon combined operation to drive two or more of the actuators, to surely feed pressure oil to all the driven actuators irrespective of the levels of loads to the driven actuators so that the two or more of the actuators can be driven at the same time.
To ensure smooth drive of all the actuators to be driven in parallel in such a hydraulic circuit as described above, it is necessary to constantly feed a hydraulic pressure sufficient to drive one of the actuators, to which a greatest load is applied. To this end, a control method, that is, the above-described load sensing control is adopted. This load sensing control means the control method that in a hydraulic circuit with actuators driven in parallel, the maximum load pressure out of load pressures under which the actuators are driven, respectively, in parallel is detected and the delivery capacity of a hydraulic pump is controlled to make the delivery pressure of the hydraulic pump higher by a predetermined value than the maximum load pressure. The adoption of this control method has made it possible not only to supply a sufficient hydraulic pressure to each actuator but also to allow the hydraulic pump to always feed a hydraulic pressure at a necessary level and hence to minimize power consumption.
A flow control system designed to perform the above-described load sensing control can feed sufficient pressure oil to each of plural actuators to surely drive the plural actuators in parallel. It can however obtained only fixed flow rate characteristics because the flow rate of pressure oil to be fed is dependent on a maximum load pressure. The driving speed is therefore determined in a wholesale manner by the above-described maximum load pressure so that, when the flow control system is employed to control plural actuators of different types, the flow control system is unable to provide flow rate characteristics conforming with the individual actuators in view of their functions and characteristics. To make it possible to obtain flow rate characteristics conforming with the individual actuators, it has heretofore been necessary for the individual actuators to design operating variable restrictors with different restricted opening characteristics and to retain flow control systems of different ratings. Taking a hydraulic excavator, for example, cylinders as actuators for a boom, an arm and a bucket are designed to be driven at appropriate speeds, respectively, because the driven elements play different roles and their preferred drive speeds are different. It is therefore necessary to mount a variety of flow control systems conforming with the characteristics of such driven elements. This has led to problems in production efficiency and production management.
In a hydraulic work vehicle driven to perform combined work at the same time, operation is not simple because the types of the work are not the same but vary substantially, because it is preferred in some instances to drive each actuator at a standard speed determined based on a stroke of the control lever and in some other instances to perform the work at a drive speed controlled lower than the standard speed. The former operation is intended to improve the efficiency of work, while the latter operation is intended to improve the accuracy of the work or the controllability. Although it is necessary for hydraulic work vehicles of this type to improve the efficiency of work and also the controllability, the conventional flow control systems which can provide only fixed flow rate characteristics are difficult to meet these two demands.
Hydraulic excavators, for example, are used in increasingly diversified ways and are equipped with an increasing number of attachments in recent years due to the diversification and complication of their working sites and enlargement of their application fields. A substantial progress has hence been made in providing hydraulic excavators with many functions. As a result, a majority of work done by a hydraulic excavator has changed from standard work having importance in the efficiency of work such as excavation of the ground or loading of earth or sand so excavated to more complex work or work requiring accuracy. It is hence required to improve not only the work efficiency but also the controllability. It has hence become more important to improve the fine controllability so that the drive speed of each actuator can be increased or decreased little by little while maintaining the drive speed at a reduced speed through a control lever. To achieve this, it is necessary to enlarge an operation range of the control lever, in which fine control of the control lever is feasible, by reducing an increase or decrease in the drive speed of the actuator per unit stroke of the control lever. The conventional flow control systems, which are designed primarily for standard work and can obtain only fixed flow rate characteristics, however cannot meet such a need.
Besides the flow control systems described above, the technique disclosed in WO 92/04505 is also known as a flow control system of the above-described type.